The invention relates generally to vascular repair devices, and in particular to endoprostheses, more commonly referred to as intravascular stents, which are adapted to be implanted into a patient's body lumen, such as a blood vessel or artery, to maintain the patency thereof. Stents are particularly useful in the treatment of atherosclerotic stenosis in arteries and blood vessels. More particularly, the present invention is directed to an intravascular stent that has a pattern or configuration that permits the stent to be placed in body vessels which are susceptible to physiological deformations and provides a high degree of fracture and fatigue resistance to such deformations.
Peripheral Artery Disease, or PAD, is characterized by fatty plaque build-up in the arteries of the legs, which results in poor blood flow and circulation. Patients with PAD may experience muscle pain during walking, have wounds and ulcers that are slow to heal or, in the most severe cases, require amputation of the legs. Possible treatments for PAD include lifestyle modification (including cessation of smoking), medicines, balloon dilatation, metal stent placement or bypass surgery.
Stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel or other body lumen such as a coronary or peripheral artery. They also are suitable for use to support and hold back a dissected arterial lining that can occlude the fluid passageway. At present, there are numerous commercial stents being marketed throughout the world. While some of these stents are flexible and have the appropriate radial rigidity needed to hold open a vessel or artery, there typically is a tradeoff between flexibility and radial strength.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metals or super-elastic nickel-titanium (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Such stents manufactured from expandable heat sensitive materials usually allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent.
Stents can be implanted in the coronary arteries along with peripheral arteries, such as the renal arteries, the carotid arteries and in long arterial segments in the leg, all of which are susceptible to arteriosclerosis. Generally, balloon-expandable stents have been implanted in the coronary arteries since the coronary arteries are generally not vulnerable to bending and compression forces that can distort the stent structure. Typically, balloon-expandable stents are made from a stainless steel or cobalt-chromium alloy, multi-layer materials or other similar biocompatible materials. Peripheral vessels, on the other hand, are usually more prone to natural bending and compressive forces which can easily bend and distort the implanted stent, causing it to fracture. For this reason, self-expanding stents are usually implanted in peripheral vessels since the self-expanding properties of the stent allows it to spring back to shape even after being subjected to bending or compressive forces.
Peripheral stents can be much longer than coronary stents since longer segments of the peripheral artery are usually required to be treated. The current trend for manufacturing peripheral stents is moving towards a longer stent, typically about 80-120 mm and longer, to treat long arterial segments in patients with critical limb ischaemia (CLI) in such arteries as, for example, the superficial femoral artery (SFA), along with arteries below the knee. Long segments of the peripheral arteries, such as the ilio-femoral-popliteal artery, usually have regions where bending and compressive forces are so constant and repetitive that even a self-expanding stent can be subjected to possible deformation caused by fatigue and fracturing. Other regions of peripheral arteries are subject to compressive forces which can prevent the stent from possibly spring back to its open, expanded configuration which can lead to stent fracture as well. For example, it has been shown that the ilio-femoral-popliteal segment undergoes non-pulsatile deformations which will, in turn, act on any stent implanted in this arterial segment. These deformations have been identified as being axial, torsional and/or bending and specific segments of the superficial femoral artery have been associated with specific non-pulsatile deformations. These deformations can impinge on the stent's ability to maintain these arteries in a fully opened position and can result in deformation and fracturing of the often intricate strut patterns. Moreover, while one stent pattern may be suitable for a particular segment of artery, the same stent pattern may not be suitable for implantation in an adjacent arterial segment if a different type of non-pulsatile deformation is present in the adjacent arterial segment.
In many procedures which utilize stents to maintain the patency of the patient's body lumen, the size of the body lumen can be quite small which prevents the use of some commercial stents which have profiles which are entirely too large to reach the small vessel. Many of these distal lesions are located deep within the tortuous vasculature of the patient which requires the stent to not only have a small profile, but also high flexibility to be advanced into these regions. As a result, the stent must be sufficiently flexible along its longitudinal axis, yet be configured to expand radially to provide sufficient strength and stability to maintain the patency of the body lumen. Moreover, the stent and its delivery system must possess sufficient axial strength to achieve the needed pushability to maneuver the stent into the area of treatment.
What has been needed and heretofore unavailable is a stent which has a high degree of flexibility so that it can be advanced through tortuous passageways and can be radially expanded in a body segment which is susceptible to physiological deformations, and yet possesses sufficient mechanical strength to hold open the body lumen or artery to provide adequate vessel wall coverage while attaining a high degree of fracture and fatigue resistance. Such a stent should be able to match the physiological deformations associated in various regions of the body vessel to effectively provide a high level of fracture and fatigue resistance to the various loading conditions and deformation patterns to which the stent may be subjected. The present invention satisfies these and other needs.